Pressure sensor

ABSTRACT

A semiconductor strain gauge is mounted on a cantilever whose free end is moved by a diaphragm or bellows adapted to be displaced by a pressure in a pressure chamber which varies, whereby the semiconductor strain gauge can produce an electric signal in accordance with the pressure. The diaphragm and bellows are constructed such that they produce a counter balancing force which increases in coefficient in accordance with an increase in the amount of its displacement.

United States Patent [191 Shimada et al.

1 PRESSURE SENSOR [75] Inventors: Satoshi Shimada, Hitachi; IchiroKimura, Mito, both of Japan [73] Assignee: Hitachi, Ltd., Japan [22]Filed: Oct. 25, 1973 [21] Appl. No.: 409,416

[30] Foreign Application Priority Data Nov. 1, 1972 Japan 47-108832 [52]U.S. Cl 73/393; 73/115; 73/398 AR; 73/407 R [51] Int. Cl. G011 9/04 [58]Field of Search 73/393, 115, 118, 398 AR, 73/407 R, 410, 406; 267/180[56] References Cited UNITED STATES PATENTS 3,195,568 7/ 1965 Pearl267/180 3,444,736 5/1969 Steclman 73/407 R July 15, 1975 3,485,10412/1969 Sanford 73/407 R 3,611,711 10/1971 Mueller 73/407 3,780,58812/1973 Whitehead, Jr. et a1 73/398 AR FOREIGN PATENTS OR APPLICATIONS635,003 2/1962 ltaly 267/180 Primary Examiner-Jerry W. Myracle AssistantExaminer-Marcus S. Rasco Attorney, Agent, or FirmCraig & Antonelli [57]ABSTRACT 2 Claims, 6 Drawing Figures FIG.

SHEET FIG. 3

FIG. 2

F'IKTSNTEUJLH 15 9 5 w 8 4 sum 2 v0 Pm(mmHgobs) 760 PRESSURE SENSOR Thisinvention relates to pressure sensors, and more particularly it dealswith a pressure sensor adapted to produce a condition signal used forcalculating the volume of fuel supplied to an internal combustion engineof a motor vehicle.

An electronically controlled .fuel injection supply device is usednowadays for supplying fuel to an internal combustion engine of a motorvehicle. This device is capable of controlling fuel supply to the enginewith a higher degree of precision than a Venturi type carburetor or amechanical fuel injection supply device. Thus the device seems to bidfair to succeedin reducing noxious components of exhaust gases; Thedevice comprises an electronic control circuit which receives as itsinput an electric signal consistent with the size of a atmospheric ornegative pressure in a suction manifold in order to calculate the volumeof fuel supplied to the engine. Thus the device must be provided with apressure sensor which is capable of measuring the negative pressure inthe suction manifold and converting the same into an electric signalwith a high degree of precision.

For example, a decrease in atmospheric pressure at high altitude resultsin a reduction in air density, so that a change is caused to occur inthe air-fuel ratio. Thus, in order to maintain the air-fuel ratio at anoptimum level, it is necessary to detect the atmospheric pressure andmodify fuel supply accordingly. An optimum output condition can beobtained for an atmospheric pressure reference range and an absolutepressure reference range by varying output characteristics. It is required to obtain a smooth change in characteristics between the tworanges.

One of the objects of this invention is to provide a pressure sensorwhose output characteristics vary depending on the value of a pressure.

Another object of the invention is to provide a pressure sensor whoseoutput characteristics vary smooth in accordance with the value of apressure.

Another object of the invention is to provide a pressure sensor whichhas an output characteristic based on atmospheric pressure and an outputcharacteristic based on absolute pressure.

Another object of the invention is to provide a pres sure sensor whichemploys a semiconductor strain gauge.

Other and additional objects and features of the invention will becomeevident from the description set forth hereinafter when considered inconjunction with the accompanying drawings.

The outstanding characteristic of the invention is that means isprovided for converting the amount of displacement of pressure receivingmeans. so that the coefficient of a reaction produced by the pressurereceiving means when a pressure is brought to bear thereon can besmoothly varied according to the amount of displacement of the pressurereceiving means.

FIG. 1 is a vertical sectional front view of the pressure sensoraccording to the invention;

FIG. 2, FIG. 3, FIG. 4 and FIG. 5 are vertical sectional front viewsshowing other embodiments of the atmospheric pressure receiving means;and

FIG. 6 is a diaphragm showing output characteristics.

In FIG. 1, IA, 1B and 1C are casing members forming a casing in whichdiaphragms 2 and 3 are mounted. Defined between the diaphragm 2 and thecasing member 18 is an atmospheric pressure chamber 4 which ismaintained in communication with atmosphere through a port 5 tointroduce atmospheric pressure P/I thereinto. Defined between thediaphragm 2 and the casing member 1C is a sub-atmospheric or negativepressure chamber 6 which is maintained in communication with a manifold(not shown) of the internal combustion engine through a conduit 7 so asto introduce a negative pressure Pm thereinto from the manifold.

The diaphragm 2, which is exposed to the atmospheric pressure P11, is ofconcentric wave shape in cross-section and capable of converting apressure into a force which is proportional to the force. Mounted in thecentral portion of the diaphragm 2 is a rigid body 8 into which anadjusting screw 9 is threaded,

An airtightseal is, of course, provided between the diaphragm and therigid body and the rigid body and the adjusting screw.

A rigid body 10 is mounted in the central portion of the diaphragm 3,and a small bellows 19 is mounted between the two diaphragms 2 and 3. Anegative pressure chamber 6' defined between the diaphragm 2 and bellowsI9 is maintained in communication with the negative pressure chamber 6through a duct 20. A vacuum chamber 11 is defined between the diaphragm3 and bellows I9, and a cantilever 13 fixed at its base to the casingmember 1A by a screw 12 extends into the vacuum chamber 11. A steel ball14 is mounted at a free end of the cantilever 13 which is disposed on animaginary line connecting the adjusting screw 9 to the rigid body I0.The adjusting screw 9 is maintained at its forward end in rotatableengagement with the steel ball 14 through the bellows 19. The free endof cantilever 13 is connected to the rigid body 10 through a flexiblepiano wire 15.

The relative positions of the cantilever l3 and diaphragm 3 are set suchthat the adjusting acrew 9 is tightened and the cantilever l is urged tomove a distance corresponding to a correction to be effected tocompensate for the effect of a fall in the atmospheric pressure (thatis, the cantilever 13 is moved such that the forward end of theadjusting screw 9 will not be brought out of engagement with the freeend of cantilever 13 when the atmospheric pressure falls to an expectedlevel).

A semiconductor strain gauge 16 is mounted substantially in the middleof cantilever 13 to be disposed on opposite sides of the cantilever 13.An electric signal produced by the strain gauge 16 is taken out throughan airtight glass terminal 17. An output signal produced by theaforesaid biasing of the cantilever 13 is set at a desired level bymeans of an external circuit.

The casing member 1A is formed with a stopper 18 for regulating themovement of the diaphragm 2, the surface of a portion of the stopper 18juxtaposed against the diaphragm 2 being of the same wave shape as thediaphragm 2. The diaphragm 2 and stopper 18 are spaced apart from eachother such that the clearance in an outer marginal portion is smallestand grows larger in going toward an inner peripheral portion.

In the pressure sensor constructed as aforementioned. when the pressureis in a range in which the difference between the atmospheric pressurePh in the atmospheric pressure chamber 4 and the pressure Pm in themanifold is small and the diaphragm 2 is not in contact with the stopper18, the force exerted on the cantilever 13 to move the same is the sumof a force with which the diaphragm 2 pushes the cantilever through theadjusting screw 9 and a force with which the diaphragm 3 pulls thecantilever through the piano wire 15. At this time, an electric signalvoltage e can be calculated as follows:

A (Pm o) A (Ph Pm) +2A,, (o Pm) '2S (I) Since A A1),

A Ph A,,Pm aA,,Pm 28 A,,{Ph PM (I 01)} x ES A iP/I Pm (l 11)} ES (3) eMN) KA g Pli -Pm (l +01) 2s (5) Where is S!) BS!) S]; S A The effectivearea of the diaphragm 2. 01A,, The effective area of the diaphragm 3. AThe effective area of the bellows l9. 5,, The rigidity of the diaphragm2. BS The rigidity of the diaphragm 3.

5,, The rigidity of the bellows 19.

S The rigidity of the cantilever 13.

K The coefficient of conversion of the amount displacement of thecantilever into the electric signal.

x The amount of displacement of the cantilever.

It will be evident from these formulas that the electric signal voltage6 is related in value to P/l-PIH. Thus the characteristic of the range Ashown in FIG. 6 is produced, so that correction can be effected inaccordance with the atmospheric pressure P11 (which may vary dependingon the altitude). The output voltage 6 of the strain gauge 16 is made torise when the forward end of the cantilever 13 moves from left to rightin FIG. 1, so that the atmospheric pressure detecting diaphragm 2 is outof engagement with the stopper 18 in a pressure range in which themanifold pressure Pm is near the atmospheric pressure P]: in value.Thus, when the motor vehicle is at a high altitude and the atmosphericpressure Ph falls (for example, to Ph=750 mmHg abs. the diaphragm 2moves leftwardly in FIG. I, so that the output voltage e falls and showsa change which follows the curve of Plz=750 mmI-Ig abs. in FIG. 6,thereby correcting the air-fuel ratio to compensate for the effect ofaltitude on air density.

When the value of PlzPm increases (or Pm becomes much smaller than P/l),the outer peripheral portion is brought into engagement with the stopperl8 and the force exerted on the cantilever 13 decreases as the effectivearea of the diaphragm 2 decreases. That is. the force exerted on thecantilever decreases gradually as follows:

Where K is the constant.

The rigidy of the cantilever also increases gradually.

so that the output voltage 0 shows a change as expressed by thefollowing formulas:

From formula (4),

This pressure region corresponds to the region C in FIG. 6. That is, theoutput voltage e changes very smoothly in sensitivity, and consequentlythe manifold pressure changes smoothly or the output torque changessmoothly when the acceleration pedal is depressed, thereby contributingto an increase in the riding comfort of the vehicle at the time ofacceleration or deceleration.

When the manifold pressure Pm further decreases and the diaphragm 2 isbrought into full engagement with the stopper 18 so that the manifoldceases to function, it is only a force proportional to 0Pmm and actingon opposite sides of the diaphragm 3 that thereafter causes thecantilever 13 to move. The output voltage 0 in this ppressure region canbe expressed by the following formula:

(1A,, ES At this time, the value of S is the sum of the rigidity ofdiaphragm 3 and the rigidity of cantilever 13 and smaller than the valueof the denominator of formula (9). However, by giving a small value tothe ratio of the area of one diaphragm to that of the other diaphragm indesigning the pressure sensor, it is possible to produce a proper fallin sensitivity (a fall represented by the gradient of the curves in FIG.6). The output characteristic of this pressure range is that of therange B in FIG. 6.

As aforementioned, the change is slow in the boundary between the rangesA and B and thus the change in output takes place smoothly. Therefore,by incorporating the invention in a motor vehicle for effecting thecontrol of fuel supply to the engine, it is possible to advantageouslycontrol the air-fuel ratio at high altitudes and to accelerate ordecelerate the vehicle by operating the accelerator without impairingriding comfort.

FIG. 2 and FIG. 3 show the relation between the diaphragm 2 and stopper18. In FIG. 2, the diaphragm 2 is of wave form in cross-section and asurface portion of the stopper 18 juxtaposed against the diaphragm isplanar. In FIG. 3, the diaphragm 2 and stopper 18 are both planar insurface.

In FIG. 4 and FIG. 5, the stopper I8 is replaced by an unequal pitchcoil spring 2]. The coil spring 21 is used with the diaphragm 2 in FIG.4 and with a bellows 22 in FIG. 5. In this embodiment. convolutions ofthe spring are brought into engagement with one another starting withthe smaller pitch convolutions as the diaphragm 2 or bellows 22 isdispaced, and the effective number of windings is reduced (and thus therigidity of the spring is increased). As a result. the signal voltage eundergoes a change as follows:

e=K Pm (l0) the surface of the cantilever. an atmospheric pressurechamber exposing one side of the larger diameter diaphragm toatmosphere. and a chamber exposing the other side of each of said twodiaphragms to an engine manifold.

2. A pressure sensor according to claim 1 wherein the diaphragm exposedto atmosphere is adapted to be brought into engagement with a diaphragmstopper when it is displaced. the former being brought into engagementwith the latter first at their outer peripheral portions and the area ofcontact between them gradually extending into their inner peripheralportions.

1. A pressure sensor comprising two diaphragms differing from each otherin effective diameter, a bellows interposed between said two diaphragms,a vacuum chamber defined by said bellows and the smaller diameterdiaphragm, a cantilever disposed in said vacuum chamber and adapted tobe moved by said two diaphragms, a semiconductor strain gauge producingan electric signal consistent with the strain produced in the surface ofthe cantilever, an atmospheric pressure chamber exposing one side of thelarger diameter diaphRagm to atmosphere, and a chamber exposing theother side of each of said two diaphragms to an engine manifold.
 2. Apressure sensor according to claim 1 wherein the diaphragm exposed toatmosphere is adapted to be brought into engagement with a diaphragmstopper when it is displaced, the former being brought into engagementwith the latter first at their outer peripheral portions and the area ofcontact between them gradually extending into their inner peripheralportions.